Nigrosines and indulines are azine dyes having blue, violet and black shades. They have a considerable technical importance due to the low cost and to the variety of applications, which are the ones typical of the solvent dyes. They can be employed to color waxes, lakes, shoe-polishes, printing inks, ribbons for typewriters and plastic materials.
The indulines are the products obtained from the reaction of aniline and aniline hydrochloride with 4-aminoazobenzene. The nigrosines are the products obtained from the reaction of aniline with nitrobenzene in the presence of ferrous chloride and hydrochloric acid.
Both nigrosine and indulines are complex mixtures of dyes. The composition of these mixtures depends on the reaction conditions (melting) employed when preparing same, and in particular on the reaction duration and temperature. The higher the temperature, the more aniline residues enter the molecule and the bluer the product becomes. In "Ullmanns Encyklopeadie der Technischen Chemie" 3rd ed. (1960), vol. 12, pages 733-4, there are listed seven (7) components present in the induline melting.
The induline and nigrosine bases, by consequence, are not unitary compounds (as one can easily infer from a thin layer chromatography). As a result of their syntheses, they contain not only highly phenylated components such as, for example, anilido N-phenylphenosafranine (Nigrosine Base G), anilido N,N'-diphenylphenosafranine (Induline 3B) and dianilido-N,N'-diphenylphenosafranine (Induline 6B), but also impurities such as undesirable colored products having a lower condensation degree and a lower molecular weight (many of them contain free amino groups), oxidation products and aromatic amines.
This non-homogeneity of composition represents a remarkable drawback for the above-mentioned applications of the indulines and nigrosines.